We remove get-rollover-counter signal in favor of the "stats"
property. The "stats" property is a GstStructure with caps
application/x-srtp-encoder-stats that contains an array of
structures with caps application/x-srtp-stream.
Each stream structure contains "ssrc" and "roc" fields.
https://bugzilla.gnome.org/show_bug.cgi?id=733265
We add a new signal, get-rollover-counter, to the SRTP encoder. Given a
ssrc the signal will return the currently internal SRTP rollover counter
for the given stream.
For the SRTP decoder we have a new SRTP caps parameter "roc" that needs
to be set when a new SRTP stream is created for a given SSRC.
https://bugzilla.gnome.org/show_bug.cgi?id=726861
